A variety of blood treatment applications involve mass transfer. For instance, extracorporeal oxygenators are used by patients undergoing cardiopulmonary bypass procedures for cardiac surgery and by adult and infant patients with respiratory problems to oxygenate the blood and remove carbon dioxide therefrom. One type of extracorporeal oxygenator which has become relatively successful is the membrane oxygenator. Generally, in these types of oxygenators a flow of blood and a source of oxygen are separated by a semi-permeable membrane which allows the oxygen to diffuse into the blood and which also allows carbon dioxide within the blood to diffuse into the oxygen source.
There are two primary factors which affect the mass transfer rate in a membrane oxygenator or more specifically which provide resistance to the desired mass transfer, namely the diffusion resistance of the membrane material itself and the diffusion resistance of the blood (there is also a mass transfer boundary layer resistance to the transfer of carbon dioxide out of the blood which may be on the blood and/or gas side). Significant advances have been made in the development of suitable membrane materials with low diffusion resistance, and comparatively the diffusion resistance of the blood adjacent to the membrane is in fact significantly greater than the membrane diffusion resistance. This is true regardless of the configuration of the membrane (e.g., whether of a tubular, flat sheet, or pleated sheet configuration). In extracorporeal membrane oxygenators, this blood diffusion resistance is significantly more than in the natural capillaries of a human being since the size of the oxygenator blood channels is significantly greater than the size of the natural capillaries. Therefore, reducing the diffusion resistance of the blood, or more particularly the resistance provided by the mass transfer boundary layer of blood adjacent the membrane, has been the object of significant development efforts in membrane oxygenator design.
Passive augmentation techniques have been investigated for reducing the mass transfer boundary layer resistance and thus increasing the mass transfer rate in membrane oxygenators. Generally, passive augmentation utilizes the energy of the flow of the blood to induce secondary flows or eddy-type currents adjacent the membrane in order to reduce the resistance to mass transfer. More specifically, the membrane is configured such that blood flowing thereby is effectively forced to mix with more interiorly positioned portions of the blood flow. For instance, the membrane may be furrowed or a mesh may be positioned adjacent a substantially smooth membrane surface to induce an eddy-type mixing. Moreover, an eddy-like secondary flow may also be induced by utilizing a coiled tube configuration as the blood-receiving membrane in which case the curvature of the coiled tube produces the noted secondary flows.
Active augmentation techniques have also been investigated for reducing the mass transfer resistance and thus increasing the mass transfer rate in membrane oxygenators. Generally, active augmentation differs from passive augmentation in that external energy is applied in some manner to the oxygenating system. One type of active augmentation technique which has been used in some oxygenators is a pulsing of the flow of blood past a stationary membrane. In this case, a substantial portion of the energy applied to the system is concentrated on the interior portions of the blood flow (i.e., pulsing the blood flow does not concentrate the energy at the mass transfer boundary layer). Other active augmentation techniques which have been used at least experimentally are rotating membrane disk oxygenators in which a flow of blood is directed onto the face of a rotating membrane disk (i.e., the membrane surface is positioned perpendicularly to the blood flow), as well as oscillating torroid oxygenators in which oscillations of, for instance, the above-described coiled tube configuration are provided to further enhance the noted secondary flows.
Other blood treatment applications also utilize mass transfer principles. These include dialysis where a dialyzer includes a number of small tubes within a housing. Blood typically flows through the tubes and a typically liquid dialysate is contained within the housing and surrounds the tubes. As a result, impurities and excess fluid flow from the blood through the membrane and into the dialysate. Ultrafilters or blood concentrators utilize similar principles in removing excess fluid from the blood for concentrating the same.